The present invention relates to a piston for a hydraulically-operated clutch in which an expandable piston actuating chamber is partitioned so that only a small portion of the expandable chamber is in fluid communication with a source of pressurized fluid when the piston is disengaged, thereby improving response time for movement of the piston away from the disengaged position.
Automatic transmission clutches generally include a clutch drum (driving member) having an expandable piston-operating fluid chamber, a piston axially slidably positioned against the piston-operating fluid chamber, a clutch hub (driven member) coaxially disposed in the clutch drum, and a clutch plate pack interposed between the clutch drum and the clutch hub having one end directed to the piston. The clutch plate pack includes first and second groups of plates which are alternately juxtaposed. When the fluid chamber is fed with a pressurized operating fluid, the piston is forced to press the clutch plate pack thereby to engage the first and second groups of clutch plates. Under this engaged condition, the clutch drum and the clutch hub are united and thus can rotate together. When the pressurized operating fluid is drawn from the fluid chamber, the piston releases the clutch plate pack thereby to cancel the engagement between the first and second groups of clutch plates. Under this disengaged condition, the clutch drum and the clutch hub can rotate separately or individually.
These hydraulically-actuated friction clutches and brakes frequently include balance dams that provide accurate, consistent control of gear ratio changes. The balance dam creates a centrifugal hydraulic pressure that opposes and balances centrifugal hydraulic pressure developed on the apply side of the piston. These centrifugal pressures are caused by rotation of the entire clutch assembly. In this way, pressure effects due to the speed of rotation can be eliminated or reduced to a point where the control system pressure alone creates the force operative to engage the friction element.
There remains a need in the automatic transmission art to provide improved response time for gear shifting, particularly in those vehicles having push-button shifting available with automatic transmissions. It is desirable that a push-button actuated shift would occur in less than 0.5 second. Existing hydraulically-operated clutch mechanisms are unable to provide such a quick response.
The present invention provides a fast-acting piston for actuating a clutch assembly wherein an expandable chamber on the apply side of the piston is partitioned so that only a small portion of the expandable chamber is initially pressurized to move the piston toward the engaged position against the clutch pack.
More specifically, the present invention provides a hydraulically-operated clutch for an automatic transmission, including a clutch housing assembly with a clutch pack positioned therein. A movable piston is supported in the clutch housing assembly and defines an expandable chamber between the piston and clutch housing assembly. A source of pressurized fluid is positioned in communication with the expandable chamber to effect movement of the piston between engaged and disengaged positions with respect to the clutch pack. A partition structure is configured to partition the expandable chamber such that only a small portion of the expandable chamber is in fluid communication with the source of pressurized fluid when the piston is in the disengaged position, and the entire expandable chamber is in fluid communication with the source of pressurized fluid when the piston is in the engaged position, thereby providing improved response time for movement of the piston away from the disengaged position.
The clutch also includes a balance dam member defining a balance dam chamber between the piston and the balance dam member. The piston includes a port connecting the balance dam chamber with the expandable chamber, and a check valve is positioned in communication with the port.
In a preferred embodiment, the check valve is a movable ring which is juxtaposed against a plurality of openings in the piston to selectively block the openings to discommunicate the expandable chamber from the balance dam chamber.
Preferably, the clutch housing assembly includes an annular ring which is engageable with the piston to function as a partition structure to divide the expandable chamber to form the small portion of the expandable chamber. The annular ring is positioned on a clutch chamber hub member. Also, the piston includes a substantially cylindrical radially inward portion which cooperates with the ring to define the small portion of the expandable chamber.
The invention also contemplates a method of actuating a piston in a hydraulically-operated clutch, as described above, wherein the small portion of the expandable chamber is communicated with the source of pressurized fluid when the piston is in the disengaged position, and the entire expandable chamber is in fluid communication with the source of pressurized fluid when the piston is in the engaged position.
Accordingly, an object of the invention is to provide a hydraulically-operated clutch for an automatic transmission with improved response time for actuating the clutch.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.